Predicting fluid flow and transport behavior in unsaturated, fractured rock is greatly simplified where matrix imbibition can be modeled as a linear function of the square root of time (t1/2); however, such treatment implicitly assumes homogeneous matrix properties. To investigate matrix heterogeneity effects, we perform a simple experiment in which x-ray imaging is used to measure the imbibition of water from a flowing fracture into a slab of volcanic tuff. Experimental results show matrix imbibition to follow a linear t1/2 relationship even though the saturated hydraulic conductivity of the tuff varies by over four orders of magnitude. ¿ American Geophysical Union 1995